The concept of initially injecting molten thermoplastic into a mold cavity having surface temperatures greater than the glass transition temperature (Tg)--for an amorphous thermoplastic polymer--or above the melting temperatures (Tm) for a--crystalline thermoplastic polymer--is not by itself novel, but methods for practical and commercial use are believed to be scarce. No known commercial use has been made specifically for optically transparent thermoplastics injection molded into optical disks for information storage and holographic imaging, and/or optical lenses and reflective optical elements. Yang (U.S. Pat. No. 4,390,485 issued Jun. 28, 1983) teaches to "quickly heat a mold cavity surface . . . just prior to the injection of a foamable plastic resin" . . . using thin metal surface sheets heated by electrical resistance at low voltage in an otherwise conventional mold and molding sequence.
Hendry (U.S. Pat. No. 4,390,486 issued Jun. 28, 1983 is another approach to preheating mold cavity surfaces in order to create a resin-rich smooth surfaced foam molding process. Hendry uses a condensing vapor, such as steam, introduced into the mold before injection starts, in order to warm the surfaces, then tries to remove the condensing vapor before injection starts. Obviously, any residual condensate acts as an impurity at the molded surface, which would quickly produce a cosmetic flaw or even worse, in the case where a water-sensitive plastic such as polycarbonate, PET, or nylon would be brought in contact with any residual moisture, which would then cause instantaneously the well-known "silver streak" type of cosmetic flaws.
These prior art references seek to warm substantially all the part forming surfaces of the mold cavity to the extent possible, without special regard to localized heating or cooling of certain portions of the mold cavity.
Depcik (U.S. Pat. No. 5,061,415 issued Oct. 29, 1991) teaches use of localized heating of only a portion of the mold cavity by means of high frequency electromagnetic field and application of pressure in the critical region in order to prevent what he called "sunk spots" (which are believed to be what is commonly called "sink marks").
A more substantial technical disclosure, using different apparatus and approaches, is taken by Suh (U.S. Pat. No. 4,338,068 issued Jul. 6, 1982 and U.S. Pat. No. 4,548,773 issued Oct. 22, 1985). Suh recognizes that, for reproducible precise dimensions of the molded part, uniform shrinkage of the molded plastic is essential. Therefore, in a typical thermoplastic molding of cross-sectionally non-uniform wall thickness, there will inevitably be non-uniform shrinkages unless these localized regions of the mold have differential heat transfer capabilities. Suh provides this capability of localized differential thermal conductence by means of heat pipes installed at those stragegic points. Suh also teaches use of a thin mold face which can be electrically heated by means of incorporating electrical resistence wires within an electroform.
Wada (U.S. Pat. No. 4,340,551 issued Jul. 20, 1982) seeks to heat only the mold cavity surfaces, to the extent possible, to a temperature above the heat distortion of the thermoplastic resin composition, in order to again create a resin-rich layer when molding a filled or fiberglass reinforced thermoplastic. Wada uses electrical induction heating, supplied by means of a double-faced removable block (heated inductively from within) having on each of the 2 faces side a mating contour which can mate with the contour of the opposing inner facing mold cavity surfaces. This block can be moved in and out of the mold from within the parting line on each molding cycle.
Specifically, when the previous molding cycle ends and the mold opens to discharge the molded plastic part, then Wada inserts this mating contoured block into the open mold then closes the mold to warm the opposing inner facing mold cavity surfaces at a very high heating rate (degrees per second). Then after the desired mold surface temperature has been reached, the molding machine again retracts its movable platen to open the mold and withdraw the induction heating block out of the mold cavity and again close the mold at the parting line and then inject into the enclosed preheated mold the reinforced thermoplastic. The mold cavity surface temperatures being above the solidification temperatures of the plastic melt prevents surface layers of the injected melt from prematurely "skinning" or "setting up", thus providing for a much higher gloss surface than would ordinarily be the case with a random-filled mix of plastic resin and fiberglass or filler. Wada also claims benefit in reducing the weld lines formed when opposing meltfronts are joined at . . . "a conflux line in a form of thin groove having a depth of 3-5 micron or more and a width of over 10 micron". Cosmetic flaws such as flow marks, silver streak, and jetting are claimed to be eliminated by Wada's invention. Wada's alternative embodiment is to incorporate the induction heating element (B and B' as shown in FIG. 4) directly behind the mold cavity element which forms the outward facing surface on either side of the parting line. C and C' are insulation layers made from non-magnetic metals such as the alloys of copper and aluminum, which are not responsive to high frequency induction heating, in order to thereby minimize dissipation of this energy into the mold as a whole. Importantly, whereas these metals are "dead" with respect to energy flux of induction heating, they would be of course among the highest thermally conductive metals and are therefore the exact opposite of isolating the rest of the mold from thermal conduction.
Cooling means for circulating coolant within Wada molds are not shown in any of the drawings, but are implied in the text, and in example 1 Wada states that during induction heatup phase of the molding cycle, circulation of the coolant is stopped.
A more substantial limitation of Wada's preferred embodiment, however, is that the heating efficiency is directly proportional to the surface area of contact between the induction heating element (inserted at the open parting line, then clamped together between the mold halves) and the opposing mold cavity faces that are to be heated up before injection starts. These opposing mold cavity faces are, in turn, inevitably scratched and marred by this contact. The greater the clamping force applied (to bring the surfaces into intimate contact), the worse the surface damage. Any slight misalignment of the A and B moldhalves (i.e. not exactly co-axial) will aggrevate this damage, if the cavities are contoured (as would be the case with optical lenses). It is believed that is why Wada is apparently being used only for opaque (non-optical) moldings, and especially suited for textured moldings and glass-fiber-filled plastics. See enclosed Asahi publications and June 1992 Modern Plastics Magazine article (p.21). There is no support for any belief that Wada has been used successfully within the relevant field of optically transparent thermoplastics injection molded into optical disks for information storage and holographic imaging, and/or optical lenses and reflective optical elements.
Other prior art doesn't teach prewarming the mold surface above solidification temperatures of the plastic but may have relevant elements.
Waters (U.S. Pat. No. 4,623,497 issued Nov. 18, 1986) provides a passive mold cooling and heating method which locates a heating OR a cooling fluid which is capable of phase change into vapor and out of vapor by subsequent condensation. This fluid reservoir is located below the height of the mold within which circulation is to take place, so that during heat up phase, the fluid is turned into vapor and rises throughout the channels provided in the mold. Then, after condensing, the vapor returns by gravity. However, Waters' main advantage appears to be that it is a passive system, not requiring large volumes of pumped liquids in circulation to achieve its result. Nowhere does Waters teach an advantage to preheat the mold surfaces above a solidification or heat distortion temperature of the plastic, then change to cooling conditions for rapid heat removal within the same injection molding cycle. In fact, Waters is intended to be used in many forms of thermoplastic and thermoset molding, including injection molding, blow molding, rotational molding, compression molding, reaction injection molding, etc. The latter two are predominantly employed with a thermoset, crosslinkable plastics, and in those cases, the purpose of circulating heat transfer fluid is to heat the mold, not to dissipate the heat of injected molten thermoplastic. In other words, Waters does not teach alternating within the same molding cycle a heating phase and a cooling phase; he concentrates rather on the simplification of the fluid heat transfer media conveyance system.
Steinbiechler (U.S. Pat. No. 4,902,454 issued Feb. 20, 1990) provided for a intermittent actuation of certain valving, preferably in response to temperature sensors mounted within the connecting lines for which circulating coolant flows into and out of a mold temperature controlling unit on its way into and out of the molds. Preferred embodiment also employs an additional temperature sensor mounted within the mold itself.
The purpose of this invention appears to be to optimize the uniformity of mold thermal control within a tolerance band during "steady state" production injection molding of thermoplastics. That is, once startup phases are ended, and production operations are implemented, the objective is to minimize mold temperature excursions by means of opening and closing the valve sets under a computer controlled sequence. It is not the intention of Steinbiechler to warm the mold surfaces above solidification temperatures before injection and subsequently to convert to a cooling phase until solidification of the filled plastic is achieved.
Wieder (U.S. Pat. No. 4,354,812 issued Oct. 19, 1982, and U.S. Pat. No. 4,420,446 issued Dec. 13, 1983) teaches method and apparatus for automatic control of a fluid cooled plastic injection mold, to provide fastest possible cycle times by intermittent opening and closing a valve which imparts a burst of cooling fluid at a temperature substantially below the tolerance band width. However, such flow is not continuous, it is instead a series of pulsed bursts of fluid discontinuously injected into the coolant lines as directed by the computer control in response to a temperature sensing probe mounted within the mold. In this way, Wieder minimizes excursions by the mold outside of the tolerance band of the desired mold temperature setpoint. The burst of cold (substantially below the desired mold temperature setpoint) fluid is injected only after the temperature sensor sensed that the mold is now filled with hot molten plastic. Wieder does not teach preheating the mold on each cycle to a temperature above solidification temperatures of the plastic nor that any benefit would be achieved thereby. Schrammel (U.S. Pat. No. 4,731,013, issued Mar. 15, 1988) teaches thermoplastic injection molding of optical data storage disks with optically polished mold cavity surfaces which form the disk, and these mold cavity inserts are made from non-metallic materials such as zirconium oxide or silizium nitride ceramics. This reference is relevant to optical disk molding, but it only briefly mentions thermodynamic aspects of the molding (column 4, lines 10-15), and its choice of a ceramic material in place of a conventional tool steels for the mold cavity construction reduces--rather than improves--the heat transfer rate and thus would be poorly suited for use in an injection molding process wherein mold surface temperature must flucuate greatly from initial temperature above Tg to a lower temperature for solidification of the molten plastic.